What feature allows external users to share records with other users in a community?

The feature that allows external users to share records with other users in a community is Sharing Sets. Sharing Sets enable you to define a set of records that can be shared with community users based on their profile or other criteria. This is particularly useful in a community context where you want to manage access to records between different external users effectively.

When external users, such as partners or customers, are part of a community, they require a mechanism to share specific records without the complexities of the broader Salesforce sharing model. Sharing Sets provide a streamlined way to grant external users access to particular object records that are related to them or their accounts. This helps in maintaining both data security and accessibility, ensuring that users can collaborate without compromising sensitive information.

While Sharing Rules, Record Ownership, and Share Groups are all important features within Salesforce, they do not directly pertain to the ability for external users in a community to share records with each other in the same straightforward manner that Sharing Sets accomplish.